This invention relates to a continuous process for the preparation of stable, thixotropic high-internal-phase-ratio emulsions (HIPREs) having non-Newtonian flow characteristics, to an improved apparatus for use in the production of HIPREs on a continuous basis, and to methods of use thereof. More particularly, the present invention relates to improvements in structurizing elements, or blades, which are adapted to be utilized in such apparatus to provide sufficient mixing of phases to produce HIPREs on a continuous basis.
An emulsion is defined as a continuous liquid phase in which is dispersed a second, discontinuous liquid phase. When one liquid phase is introduced with agitation into another liquid phase with which is immiscible, the introduced liquid phase will disperse into discrete droplets. If the two liquid phases are pure, the droplets will begin to coalesce when agitation is stopped and two discrete layers will form. If, however, appropriate surface active materials, generally referred to as emulsifiers, are present in the system, coalescence will be prevented such that when agitation is stopped a layer of droplets of the dispersed phase will form. If the droplets of the dispersed phase, or internal phase, are small enough so that thermal and Brownian forces overcome the settling effect of the gravity field, then a stable emulsion results.
As recently as about 1967, it was believed that emulsions containing above about 75% by volume of internal phase could not be prepared due to theoretical considerations of structural packing densities and the failure of anyone to be able to prepare such emulsions. However, it was then discovered that by using unusual mixing techniques that emulsions having an internal phase of from 75% to over 99% by volume could be prepared. Emulsions comprising greater than 75% by volume internal phase (dispersed phase) are referred to a high-internal-phase-ratio emulsions (HIPREs). The droplets present in HIPREs are deformed from the usual spherical shape into polyhedral shapes and are locked in place. Thus, HIPREs are sometimes referred to as "structured" systems and display unusual rheological properties which are generally attributed to the existence of the polyhedral droplets. For example, when HIPREs are subjected to sufficiently low levels of shear stress, they behave like elastic solids. As the level of shear stress is increased, a point is reached where the polyhedral droplets begin to slide past one another whereby the HIPRE begins to flow. This point is referred to as the yield value. When such emulsions are subjected to increasingly higher shear stress, they exhibit non-Newtonian behavior, and the effective viscosity decreases rapidly.
When the shear rate ranges between 3,000-8,000 sec.sup.-1, the effective viscosity approaches the viscosities of the external and internal phases. At increasingly higher rates of shear, a point is reached where the emulsifying agents can no longer maintain stable films, and at this point the emulsion breaks and cannot be reconstituted readily. The yield value and shear stability point, as well as the shape of the viscosity versus shear rate curve, will vary with each particular emulsion formulation.
The "structured" nature of HIPREs, in addition to providing an explanation for the unusual rheological properties displayed thereby, also provides an explanation for the fact that special mixing methods are required in order to prepare such emulsions. If an attempt is made to mix two liquid phases of highly disparate viscosities, one finds that the mixing process is difficult and inefficient. When a small amount of low viscosity liquid is added to a mass of high viscosity liquid, it is difficult to incorporate homogeneously with conventional mixing means. Without appropriate mixing, as more of the low viscosity liquid is added, the highly viscous phase tends to break up and form a coarse dispersion in the thinner liquid. It is this fact which makes the preparation of high-internal-phase-ratio emulsions difficult and which has prevented development of successful continuous emulsification processes for materials of this type. With the correct type and degree of mixing, however, the low viscosity liquid can be adequately dispersed within the high viscosity liquid as it is added to form a stable emulsion. The original processes for manufacturing HIPREs were discontinuous processes which have economic disadvantages in a commercial production situation. These discontinuous processes invariably involved the preparation of a dispersion having a low portion of internal phase and subsequently adding additional internal phase until the emulsion contained over 75% internal phase. Such processes were cumbersome, but they could be successfully employed using conventional mixing equipment.
One attempt at developing a continuous process for the production of HIPREs is disclosed in Lissant U.S. Pat. No. 3,565,817 and is directed at achieving sufficient mixing by providing shear rates high enough to reduce the effective viscosity of the emulsified mass near to the viscosities of the less viscous external and internal phases. However, for certain types of emulsions, it is not possible to apply enough shear thereto to effect an apparent viscosity near those of the external and internal phases without going above the shear stability point of the emulsion. Low-fat spread emulsions (margarine) are examples of such emulsions. For example, the viscosities of the emulsions disclosed in U.S. Pat. No. 3,565,817 display an effective viscosity of less than 300 cps in the mixing region which, according to the disclosure therein, is about 10.sup.4 sec.sup.-1 (shear rate), while the viscosities of low-fat spreads, when extrapolating the shear rate plot thereof to the same mixing region, display viscosities of about 6,000 cps. Furthermore, although a variety of structurizing elements are capable of producing shear rates sufficient to reduce the effective viscosity of the emulsion phase to near the external and internal phase viscosities thereby allowing the phases to be mixed to a certain degree, such elements do not provide complete mixing of the phases as evidenced by the fact that there is always some non-emulsified liquid present in the prepared emulsion.
Mertz U.S. Pat. No. 3,946,994 and related Mertz U.S. Pat. No. 4,018,426 disclose a static mixer approach to the production of HIPREs. This approach, however, suffers from the fact that static mixers will not work for all product types, they lack sufficient shear to produce a high quality product, and they are very intolerant of minor process variations such as raw material delivery pressure variations.
One unusual approach aimed at the problem of continuously producing HIPREs is that of Bowling U.S. Pat. No. 3,684,251. This reference uses a series of from 7 to 21 individual mixing chambers to prepare HIPRE emulsions. In the first mixing chamber the external phase is introduced at twice the flow rate of internal phase. Thus, the first chamber produces an emulsion having only about 33% internal phase. The reference then gradually adds additional internal phase until the desired volume percent of internal phase is emulsified. The difference between this and convention batch processes, is that this references adds the additional internal phase not in the original mixing vessel, but with each addition of internal phase the mixing takes place in a new mixing vessel. Thus, this approach to a continuous process is to string together a series of 7 to 21 discontinuous processes. As can be seen from the drawings in this reference, the apparatus is exceedingly complex because of the need to separately meter internal phase into each separate mixing chamber. If any one of the 7 to 21 flow meters is not properly set, the desired emulsion will not be produced. Because of the complexity of the apparatus and its operating parameters, the possibility of failure is orders of magnitude higher than the possibility of failure of a much simpler conventional device.
It has now been discovered that complete mixing can be effected without applying sufficient shear to reduce the effective viscosity of the emulsified mass to near the viscosities of the external and internal phases. Furthermore, it has now been discovered that by providing complete mixing, the present of non-emulsified liquid in the prepared emulsion is significantly reduced or eliminated whereby improvements in the quality of emulsions, in terms of texture, is achieved. This is important in the cosmetics and food industries, as well as others, where product appearance is a major marketing factor.
Therefore, the present invention overcomes the shortcomings and disadvantages of prior art process for the production of HPREs on a continuous basis by providing an apparatus which includes a plurality of structurizing elements which are adapted to provide complete mixing of the phases.
The aforementioned U.S. Pat. No. 3,565,817 discloses an apparatus adapted to be utilized in a process for the continuous production of HIPREs including a mixing chamber equipped with mixing blades. The structure of such mixing blades is not disclosed.
U.S. Pat. Nos. 2,673,077; 2,682,276; 3,166,303; 3,207,488; 3,565,817; 3,939,073; and 4,128,342 all disclose mixing blade structures. Also note, for example, GB 841,743; DE 1,001,663; DE 2,753,153; JP 55-134634 and CS 71,479. None of the mixing blade structures disclosed in these patents are adapted to provide complete mixing of the phases to produce HIPREs on a continuous basis.
McConnaughay U.S. Pat. No. 3,284,056 discloses an apparatus for producing bituminous emulsions such as asphalt-in-water. Although called "emulsions", the products of this patent might actually be suspensions. Emulsions can be used to carry solid particles, but the concept of an emulsions requires one liquid phase dispersed in another liquid phase. In any event, this reference is definitely not directed at preparing HIPRE emulsions, and furthermore, it employs very narrow spacing between its mixing elements and would not produce acceptable HIPREs. Although the blades of this reference are apparently capable of producing a large amount of shear, shear alone is not sufficient to produce a stable HIPRE (in fact, too much shear can destroy a stable HIPRE).
The difficulty in preparing HIPREs is in part due to the unusual rheological properties of these materials. The internal and external phases of the HIPRE are themselves of relatively low viscosity, but as the emulsion is formed, the viscosity of the emulsion becomes very high. However, it is quite difficult to blend a low viscosity liquid into a high viscosity liquid. Thus, once the emulsion begins to thicken, the remaining low viscosity internal phase is very difficult to incorporate into the emulsion and the emulsion begins to break up into coarse droplets which are dispersed in the intended internal phase. It is for this reason that HIPRE emulsions have been very difficult to manufacture. If providing shear alone were sufficient, references such as the aforementioned U.S. Pat. No. 3,684,251 would not have bothered to use 7 to 21 individual mixing chambers. Rather, that reference simply would have used any conventional, high shear mixing blades.
Most so-called continuous emulsification devices which have been employed for the production of low- and medium-internal-phase-ratio emulsions are not suitable for producing high-internal-phase-ratio emulsions because they are not capable of providing sufficient deforming force to the structured systems to move the polyhedral droplets past one another and therefore do not accomplish the required mixing, or such devices produce shear rates in excess of the inherent shear stability point. Most importantly, such devices do not provide for adequate mixing of the phases particularly where there is a large disparity in the viscosities of the two phases to be utilized wherein the polyhedral droplets are locked into a structured system to a greater degree. Thus, colloid mills and other high shear devices cannot be used. Also, low shear mixing devices, such as Hobart mixers or other equipment utilizing slow moving paddle-type stirrers do not provide sufficient deforming force and therefore do not provide complete mixing of the phases to produce HIPREs on a continuous basis.